The present invention relates to a process for preparing the potassium peroxomonosulfate triple salt represented by the formula: EQU 2 KHSO.sub.5 .multidot.KHSO.sub.4 .multidot.K.sub.2 SO.sub.4
by partial neutralization of sulfuric acid-Caro's acid fed into an aqueous working solution containing KHSO.sub.5, KHSO.sub.4 and K.sub.2 SO.sub.4 using a caustic potash solution with simultaneous evaporation of water.
Due to the high oxidation potential of potassium peroxomonosulfate, the triple salt 2 KHSO.sub.5 .multidot.KHSO.sub.4 .multidot.K.sub.2 SO.sub.4 is of industrial importance as a component of bleaches, cleansing agents, detergents, etching agents and as an oxidizing agent in inorganic reactions. The triple salt should be as free as possible of by-products and be sufficiently stable in air. The term "2 KHSO.sub.5 .multidot.KHSO.sub.4 .multidot.K.sub.2 SO.sub.4 triple salt" is also understood to include those products whose composition differs slightly from the molar ratios mentioned. The molar ratio may thus also be in the range 1.8 to 2.1:1 to 0.9:1.1. The concentration of potassium peroxomonosulfate in the triple salt is generally between 45 and 50 mol. %.
Preparation of the 2 KHSO.sub.5 .multidot.KHSO.sub.4 .multidot.K.sub.2 SO.sub.4 triple salt is based on the reaction of sulfuric acid or oleum with hydrogen peroxide and partial neutralization of the sulfuric acid-Caro's acid produced thereby with basic potassium compounds such as KOH, K.sub.2 CO.sub.3 or KHCO.sub.3. The triple salt can be recovered from the partially neutralized reaction mixture by evaporating the water. In accordance with U.S. Pat. No. 3,041,139 (which is incorporated by reference in its entirety), preparation takes place in the manner described above. The disadvantage of the process described therein is the cold crystallization procedure required, which is both energy-consuming and also requires additional equipment. A further disadvantage of this known process is the fact that the yield of active oxygen in the isolated triple salt, with reference to the hydrogen peroxide used, is generally between only 50 and 60%. Finally, the triple salt prepared by this method generally contains, as a by-product, about 3 wt. % of potassium peroxodisulfate.
In the process disclosed in GB Patent 979,450, in which the 2 KHSO.sub.5 .multidot.KHSO.sub.4 .multidot.K.sub.2 SO.sub.4 triple salt is not mentioned expressis verbis, potassium hydrogen sulfate and/or potassium sulfate are added to the sulfuric acid-Caro's acid or directly to the aqueous hydrogen peroxide solution used for its production in order to promote crystallization of the salt containing KHSO.sub.5, KHSO.sub.4 and K.sub.2 SO.sub.4. An increase in yield of up to about 10% is apparently produced by the reaction of KHSO.sub.4 with H.sub.2 O.sub.2. The reaction in this known process is also performed at very low temperatures; moreover, the addition of a solid is required which increases the cost of the process.
DE 34 27 119 (U.S. Pat. No. 4,610,865 which is incorporated by reference in its entirety) discloses a continuous process for preparing the 2 KHSO.sub.5 .multidot.KHSO.sub.4 .multidot.K.sub.2 SO.sub.4 triple salt, wherein an aqueous working solution which contains KHSO.sub.5, H.sub.2 SO.sub.4 and K.sub.2 SO.sub.4 in molar ratios from 1.3 to 2.5:1.2 to 2.0:1 is concentrated in an evaporation unit under reduced pressure and at a temperature of at most 40.degree. C. to give a concentration of 25 to 30 wt. % of KHSO.sub.5. The triple salt is precipitated by cold crystallization and, after isolation of the same, the mother liquor is reconstituted by adding sulfuric acid and hydrogen peroxide as well as concentrated KOH and is used as the working solution. This process leads to a product containing very little K.sub.2 S.sub.2 O.sub.8, but in order also to produce a good yield of active oxygen, it is necessary to use expensive, very highly concentrated, preferably 85 wt. % strength, hydrogen peroxide. A further disadvantage is that a cold crystallization stage is also used in addition to the evaporation stage, which requires additional equipment and cooling energy.